1. Not finding help here? Sign up for a free 30min tutor trial with Chegg Tutors
    Dismiss Notice
Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Need help with Physics problem (momentum/collisions)

  1. Apr 7, 2014 #1
    1. The problem statement, all variables and given/known data

    A 0.800 kg target slides along the ice at 3.0 m/s [W], when it is hit by a 20.0 g arrow moving at 260 m/s [N], as part of a show. Find the final velocity of the target after the inelastic collision.

    2. Relevant equations

    pi= pf
    m1 v1i+ m2 v2i = (m1+ m2)vf
    pythagorean theorem

    3. The attempt at a solution

    v1f= ?

    pix= pfx
    m1 v1ix+ m2 v_2ix = (m1+ m2)vfx
    (0.80)(-3.0)+0=(0.80+0.020) vfx
    -2.4=(0.820)vfx
    v_fx= -2.93 m/s

    piy= pfy
    m1 v1iy+ m2 v2iy = (m1+ m2)vfy
    0+(0.02)(260)=(0.800+0.02) vfy
    vfy=5.2/0.82=6.34 m/s

    vf=√(-2.93)^2+(6.34)^2
    vf=6.98 m/s

    tanθ= -2.93/6.34
    θ=25°

    Since one of these values are negative, the angle is negative. Relatively speaking what would the angle be. also was this question in general answered correctly?
     
  2. jcsd
  3. Apr 7, 2014 #2
    Yes you used the conservation of linear momentum properly, and successfully applied it to each coordinate individually.

    The last step however is wrong, as the ratio should be:

    [itex]
    \tan{\theta}=\frac{6.34}{-2.93}
    [/itex]

    Which would give:

    [itex]
    \theta=-65.2^\circ
    [/itex]


    The negative angle interpretation depends on how you defined your angles. In this case, the angles are defined in the sense such that [itex]\theta=0[/itex] along the "negative x-axis". Therefore a negative angle just represents clockwise rotation as opposed to counter-clockwise rotation. So it is travelling [itex]65^\circ[/itex] in the North-West direction. Make sense?

    Normally, for example, [itex]\theta=45^\circ[/itex] would be above the x-axis in the 1st quadrant, whereas [itex]\theta=-45^\circ[/itex] would be below the x-axis in the 4th quadrant. In this case [itex]\theta=45^\circ[/itex] would be in the 3rd quadrant below the x-axis and [itex]\theta=-45^\circ[/itex] would be above the x-axis in the second quadrant.

    This is due to the fact that the target is moving initially at an angle [itex]-180^\circ[/itex] which is now taken as [itex]0^\circ[/itex]. Does this help?
     
    Last edited: Apr 7, 2014
  4. Apr 7, 2014 #3
    yes thanks so much, great help
     
  5. Apr 7, 2014 #4
    so would that be [W 65° N] or [N 25° W]?
     
  6. Apr 7, 2014 #5
    Yes. You did it right, but why did you assume that the target was moving in the negative x direction to start with? There's nothing wrong with this, but I would have had it going in the + x direction (for some reason). Oh well. Potatoes, Potahtoes.

    Chet
     
  7. Apr 7, 2014 #6
    If I understand what you're saying correctly then it would be [itex]\text{[W } 65^\circ \text{ N]}[/itex] (65 degrees North of West).

    Also he did it that way because it specified that it was travelling West, and in introductory physics no one expects people to think like that probably.
     
  8. Apr 7, 2014 #7
    Oh thanks. I didn't notice that [W].

    Chet
     
  9. Apr 7, 2014 #8
    yes, is [W 65∘ N] the correct angle?
     
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook

Have something to add?
Draft saved Draft deleted



Similar Discussions: Need help with Physics problem (momentum/collisions)
Loading...